There are several industrial applications known in the art, either of electrolytic or electrometallurgical nature, that make use of anodes whereupon the evolution of a gaseous product takes place, whose achievement constitutes in some cases the primary aim of the process (as for the chlorine evolved in the electrolysis of alkaline chlorides or hydrochloric acid). In other cases, the evolved gas is just a by-product of the reaction (as in the case of oxygen evolved in the processes of metal cathodic electroplating, typical of the galvanic industry). In both cases, one of the primary objects in the realisation of electrodes for gas evolution, and in particular of the anodes, is the high electrochemical activity, that must allow operating with the lowest possible overvoltages in order to increase the global energetic efficiency of the process. It is therefore common practice, also in case the gas developed on an electrode surface is just a by-product, to carry out such reactions on catalytic surfaces. Since the materials with the best electrocatalytic properties are very expensive, such a category fundamentally comprising the platinum group metals and their oxides, their employment is common only as thin superficial layers, coated on a conductive matrix. In particular, it is widely known to the experts in the art the use of metallic substrates coupling good current conduction and corrosion resistance features, having at least one surface coated with a thin layer of noble metals and/or oxides or alloys thereof; embodiments of this kind are for instance disclosed in U.S. Pat. No. 3,428,544, No. 3,711,385, and many others. The corrosion resistance of the metallic substrate is a very critical parameter especially in the case of electrodes destined to function as anodes, where the aggressiveness of the electrolytes is further favoured by the electrochemical working potential. For this reason, the anodes for industrial electrolytic and electrometallurgical applications are preferably realised starting from substrates of valve metals, that is metals resisting to corrosion for being protected by a thin superficial film of inert oxide. Among these, the metal most often employed is by far titanium, for reasons of cost and workability. The electrochemical characteristics of titanium matrixes coated with noble metal oxide based catalysts are normally considered more than satisfying as gas evolving anodes for nearly all the industrial electrochemical applications. Conversely their lifetime, especially in the most critical working conditions (highly aggressive electrolytes, very high current density, etc.) constitutes, in many cases, a problem not yet fully solved, although a rich literature exists by now testifying some fundamental progresses in this field. A high duration of the electrodes is a fundamental condition for the industrial success of the electrochemical applications, not only because, in case of deactivation, a new electrochemical coating, inherently expensive both in terms of material and of manpower, must be deposited, but also for the missed production associated to the plant shut-downs required for the replacement of the electrodes. Since the noble metals used in the formulation of electrocatalytic coatings are per se immune from corrosion in the usual operating conditions, the prevailing cause of deactivation consists in the local detachment of the coating from the substrate, with consequent corrosion or passivation of the latter. Such detachment is favoured from the gas evolution itself, due to the mechanical action of the bubbles formed on the surface, and the phenomenon is further emphasised at high current density. In particular, in some electrometallurgical applications with anodic oxygen evolution, for instance in the zinc plating of sheets for use in the car industry or in the production of thin copper sheets for use in the electronic industry, anodic current densities exceeding 15 kA/m2 are reached.
A further factor of instability for the adhesion of the coating to the substrate may derive from the porosity of the former, allowing the infiltration of electrolyte in direct contact with the unprotected metallic matrix. In such cases, in particular if zones of detachment exist even if microscopic, passivation of the substrate can occur, with formation of an often scarcely conductive oxide interposed between substrate and electrocatalytic coating, without the physical detachment of the latter taking place. To obtain a sufficient anchoring of the electrocatalytic coating to the substrate the usefulness of conferring a certain roughness to the substrate itself, for instance by means of a sandblasting treatment, or by controlled etching with a corrosive agent, is widely known since the origin of this type of electrodes. The superficial roughness favours the mutual penetration of the substrate and the catalyst, obtained through the thermal treatment of a precursor applied to the substrate in form of a paint. In the case of titanium for instance, abrasive treatments with sand, sand mixed to water or corundum, and etching with hydrochloric acid are well established; such procedures allow obtaining electrodes which find a possible use in some industrial applications, notwithstanding the necessity of submitting the electrodes to a still rather frequent periodic reactivation. Among the most penalised applications, the electrometallurgical processes with anodic evolution of oxygen should again be cited, especially in case operation at current density higher than 10 kA/m2 is required. Also for low current density processes however, as in the case of electrowinning in acidic environment of metals from solutions deriving from ore dissolution, problems subsist, albeit of a different kind; among them, the impurities always present in the electrolytic baths, some of which have an extremely deleterious effect on the passivation of titanium matrixes. A classic example is given by fluoride ions, capable of complexing titanium thereby destroying the protective film with consequent attack of the underlying metallic matrix, especially in zones where micro-defects in the adhesion of the electrocatalytic coating to the substrate are already present.
The employment of intermediate coatings with adequate characteristics of corrosion inhibition to be interposed between metallic substrate and electrocatalytic coating has been thus repeatedly proposed under different forms, so that the corrosive attack in correspondence of the always present micro-defects is stopped in correspondence of such barrier. An example of intermediate coating, based on ceramic oxides of valve metals, is disclosed in the European Patent EP 0 545 869, but several other types of intermediate coating, mainly based on transition metal oxides, are known in the art.
The definition of the optimal roughness parameters of electrodic matrixes suited to receive an electrocatalytic coating is for instance disclosed in the European Patent EP 0 407 349, assigned to Eltech Systems Corporation, USA, wherein it is specified that, in order to achieve a good quality adhesion of the coating itself, it is necessary to impart a superficial average roughness not lower than 250 microinches (about 6 micrometres), with an average of at least 40 peaks per inch (on the basis of a profilometer upper threshold of 400 microinches, that is about 10 micrometres, and of a lower threshold of 200 microinches, that is about 8 micrometres).
The finding disclosed in EP 0 407 349 constitutes a step forward toward the definition of an electrode with improved characteristics of potential and duration, however it is apparent to the experts of the field that such a high roughness, obtained by means of a severe generalised attack of the surface of chemical or mechanical nature, requires the deposition of catalytic layers of a certain thickness to obtain a sufficiently homogeneous covering. It is a customary practice, known to the experts in the art, the deposition of catalytic layers, independent of the presence of intermediate protective layers, having an overall noble metal loading well higher than 10 g/m2, preferably comprised between 20 and 30 g/m2, for all of the cited industrial (electrolytic and electrometallurgical) applications. In the absence of this, the duration of the anodes for gas evolution is still largely insufficient.
Also the subsequent patent application US-2001-0052468-A1, which provides superimposing a microrough profile on a macrorough profile quite similar to the one of EP 0 407 349, although giving electrodes with superior lifetime characteristics also in the absence of intermediate coatings, is fundamentally directed to electrodes with consistent noble metal loadings (24 g/m2 in the examples). Such high loadings of noble metal are onerous from an economical standpoint, and in some cases they are not acceptable at all: this is especially the case of the primary electrometallurgical applications (electrowinning and similar), where the added value of the products is not high enough to justify such elevated investment costs.